Closed cycle system for the generation of chlorine dioxide



CLOSED CYCLE SYSTEM FOR THE GENERATION OF CHLORTNE DIOXTDE Filed 0G12.2, 1967 Aug. 18, 1970 G. o. WESTERLUND 8 Sheets-Sheet l Nou N5 ug. 18,1970 G. o. WESTERLUND 3,524,728

CLOSED CYCLE SYSTEM FOR THE`GENERATION OF CHLORINE DIOXIDE 8Sheets-Sheet 2 Filed Oct.

Nom

mODOH A HQ" 5150 Aug. 18, 1970 G. o. WESTERLUND CLOSED CYCLE SYSTEM FORTHE GENERATION OF CHLORINE DIOXIDE 8 Sheets-Sheet 3 Filed Oct. 2, 1967Aug. 18, 1970 G. o. wEsTERL-UND CLOSED CYCLE SYSTEM FOR THE GENERATIONOF CHLORINE DIOXIDE Filed OCT.. 2. 1967 8 Sheets-Sheet 4 CLOSED CYCLESYSTEM FOR THE GENERATION OF CHLORINB DIOXIDE 8 Sheets-Sheet 5 FiledOCC. 2. 1967 can lx CLOSED CYCLE SYSTEM FOR THE GENERATION OF CHLORINEDIOXIDE Filed Oct. 2. 1967 ugv18, 1970 G. o. wEsTl-:RLUND 8 Sheets-Sheet6 Aug. 18, 1970 G, o. wEsTERLuND 3,524,728

CLOSED CYCLE SYSTEMFOR THE GENERATION OF HLORINE DIOXIDE 8 Sheets-Sheet'7 Filed ot. 2. 1967- Aug.4 18, 1970 5(l o. WESTERLUND CLOSED CYCLESYSTEM FOR THE GENERATION OF CHLORINE D IOXIDE Filed oct. 2, 1967 8Sheets-Sheet 8 United States Patent O 3,524,728 CLOSED CYCLE SYSTEM FORTHE GENERATION OF CHLORINE DIOXIDE Gothe Oscar Westerlund, Vancouver,British Columbia,

Canada, assignor to Chemech Engineering Ltd., Vancouver, BritishColumbia, Canada Filed Oct. 2, 1967, Ser. No. 675,272 Claims priority,application Canada, Oct. 5, 1966,

72,150 Int. Cl. B01k 1/00; C01b 11/00, 11/26' U.S. Cl. 23-262 18 ClaimsABSTRACT F THE DISCLOSURE A continuous recyclic process and apparatusfor the production of chlorine dioxide from an aqueous chlorationsolution of gaseous hydrochloride. The hydrogen chloride is formed insitu by combustion of hydrogen gas and chlorine gas. The chlorinedioxide is used for bleaching pulp.

This invention relates to the production of chlorine dioxide, and moreparticularly to an improved process for producing chlorine dioxidecontinuously and rapidly with high yields.

Chlorine dioxide has been prepared by treating chlorates with sulphuricacid with mixtures of sulphuric acid and an organic reducing agent suchas oxalic acid. These processes are, in general, uneconomical, hazardousand wasteful.

For example, Harry N. Tatomers process of producing chlorine dioxide,Canadian Pat. No. 452,351 issued Nov. 2, 1948, employs chloratesolution, sulphuric acid and chloride solution and generates at leastone mole of chlorine for every two moles of chlorine dioxide. The eiuentliquor is rich in chemicals which go to waste unless expensive recoveryequipment such as evaporators and crystallizers are employed, or unlessthe chemicals in the effluent liquor are utilized by other processes.

Processes using a metallic chlorate and mixtures of a strong mineralacid and reducing agents such as sulfur dioxide, chromic acid, nitrogenperoxide, alcohols and aldehydes yield chlorine dioxide with lowerchlorine concentrations. However, the eiiciency or yield of chlorinedioxide is not as high and the reagents are more expensive. CanadianPat. No. 533,803, issued Nov. 27, 1956 to Francis H. Dole uses sulphurdioxide in a mixture Of a sulphuric acid and sodium chlorate solution.Another example is Henry C. Marks et al., U.S. Pat. No. 2,616,792 issuedApr. 1, 1949 which uses an excess of nitrogen peroxide on chloratesolution. Still another example is S. H. Perssons Canadian Pat. No.438,316 issued Dec. 3, 1946, uses chromic sulphate on an aciditedchlorate solution. Finally, Max L. Audonauds Canadian Pat. No. 512,954issued May 17, 1955 produces a chlorine dioxide by blowing air or inertgas through a porous member into an acid solution of chlorate.

It is also well known that hydrochloric acid and chloric acid may bereacted to produce gaseous mixtures of chlorine dioxide and chlorine, asin George A. Days Canadian Pat. 461,586 issued Dec. 6, 1949 and U.S.Pat. No. 2,664,341 issued Dec. 29, 1953. In practice these reactions arecarried out by treating aqueous mixtures of water-soluble chlorates andchlorides, such as may be obtained for example by chlorinating lime orby the electrolysis of salt, with an excess of a strong inorganic acid,such as hydrochloric acid or sulphuric acid. The following equationsrepresent these reactions:

Ordinarily reaction (2) is predominant over reaction (l) and the yieldin chlorine dioxide is correspondingly low.

Patented Aug. 18, 1970 To minimize reaction (2) it has been suggested toreact properly proportioned mixtures of chlorates, chlorides and astrong inorganic acid in dilute solutions (containing at least 50% andpreferably up to 75% of water) at temperatures below 60 C. Based onreaction (1), equivalent ratios of Cl/ C103=2 and of H+/ Cl03=2 shouldgive high yields of C102 per mol of chlorate decomposed. In practice,however, it has been proposed in particular to use a ratio of Hf/ C103-in excess of 2 because reaction (2) uses some of the chlorate inproducing chlorine instead of C102. This proposal results in the use ofexcessive quantities of acid.

Furthermore, it has been recognized that a high yield of C102 per mol ofchlorate decomposed, while desirable, is not alone sutiicient to makethe process economical for large scale production of chlorine dioxide.As a matter of practical necessity, it has therefore been recommendedthat the decomposition of the chlorate initially present be carriedsubstantially to completion to avoid any appreciable waste of thisValuable raw material. However, the requirement of consuming all, oralmost all, of the chlorate entails inherent diiiculties which greatlydecrease the efficiency, rapidity and therefore the economy of the olderprocess. One diiculty is the fact that the average hourly output of C102is necessarily low because the reaction rate decreases considerably asthe concentration of the reactants, particularly of chlorate, decreases.The use of solutions of low chlorate content further magnies this eifectand wastes valuable space in the reaction chamber. Finally, as theconcentration of chlorate decreases, reaction (2) contributesincreasingly to the decomposition of the chlorate whereby the overallyield of chlorine dioxide is lowered.

Another prior process involves reacting solutions of chlorates withhydrochloric acid, the acid being supplied in amount substantially lessthan the equivalent stoichiometric ratio of H+/C1O3=2 of reaction (1),thereby decomposing at any one time only a fraction of the availablechlorate, said decomposition thus proceeding at a particularly rapidrate, enriching the chlorate content of the partially spent solution, asfor example by feeding it to an electrolytic chlorate cell, returningthe fortified solution to the reaction chamber to treat it again with astoichiometrically insuliicient amount of acid; and repeating thiscycle, whereby substantially all the chlorate supplied is eventuallyefficiently decomposed, producing mixtures of chlorine dioxide andchlorine containing high proportions of chlorine dioxide.

However, the process is generally unsatisfactory since it useshydrochloric acid, which is considerably more expensive than chlorineand in remote areas is prohibitive because of transportation cost.Secondly, additional expensive equipment is necessary to produce thecompressed air which is used as dilutent for the chlorine dioxide andchlorine generator gases to reduce the inherent explosion hazard of theprocess.

Again, in the aforesaid process, external heat or steam is required tovaporize hydrogen chloride and to maintain desired reaction temperaturein the gas-generators.

Finally, a minimum of one mole of chlorine is produced per two -moles ofchlorine dioxide generated. Since the raw material is hydrochloric acid,the chlorine is produced from the purchased acid. This is a largeeconomic penalty since the cost of the acid used as raw materialnormally is considerably higher than the equivalent cost of chlorine ifpurchased.

Copending application Canadian Ser. No. 906,199 filed June 30, 1964provides an improved continuous recyclic process and apparatus for theproduction of chlorine dioxide. The present invention is an improvementover the process of that application which involves the steps of (a)effecting electrolysis of an aqueous solution of a metal chloridewhereby to form (i) an aqueous solution of a metal chlorate and (ii)gaseous hydrogen; (b) reacting the gaseous hydrogen (a)(ii) ywithgaseous chlorine whereby to form (iii) gaseous hydrogen chloride; (c)reacting the aqueous solution of metal chlorate (a) (i) with the gaseoushydrogen chloride (b)(iii) whereby to form (iv) an aqueous solution ofmetal chloride, which is recycled to step (a) and (v) an aqueoussolution of chloric acid, and (d) reacting the aqueous solution ofchloric acid (c)(v) with the gaseous hydrogen chloride (b)(iii) wherebyto form (vi) chlorine dioxide, which is recovered; (vii) water and(viii) gaseous chlorine which is recycled to step (b).

An object of one aspect of the present invention is the provision of aprocess for preparing chlorine dioxide from an aqueous chlorate solutionand gaseous hydrogen chloride in which explosion hazards attendant withthe production of such gaseous chlorine dioxide are minimized.

An object of another aspect of the present invention is the provision ofa process for preparing chlorine dioxide from an aqueous chloratesolution and gaseous hydrogen chloride formed in situ by combustion ofhydrogen gas and chlorine gas in which the production capacity n in thecombustion reaction is increased.

An object of yet another aspect of the present invention is theprovision of a continuous recyclic process for the production ofchlorine dioxide in which a minimum of raw materials is necessary tomaintain the operation thereof.

An object of another aspect of the present invention is the provision ofa continuous recyclic process for the preparation of chlorine dioxide inwhich no substantial gaseous by-products are produced.

An object of yet another aspect of the present invention is theprovision of a continuous recyclic process for the production ofchlorine dioxide in which the system is self-contained in regard toheat.

An object of still another aspect of the present invention is theprovision of a continuous recyclic process for the production ofchlorine dioxide which is particularly suited for pulp mills producingbleached pulp.

An object of a still further aspect of the present invention is theprovision of a continuous recyclic process for the production ofchlorine dioxide which is simple and safe in operation and easilycontrolled.

An object of a still further aspect of this invention is the provisionof a continuous process for the production of chlorine dioxide involvingpreparation of primary reactants in electrolytic cells operating underconditions tending to minimize current eciency losses.

An object of another aspect of the present invention is the provision ofapparatus for the continuous production of chlorine dioxide.

By one broad aspect of this invention, there is provided, in a processfor converting an aqueous solution of chlorate into chlorine dioxide byreaction thereof with hydrogen chloride, the improvement of diluting thehydrogen chloride with suicient chlorine gas to provide a nal gaseousreaction product comprising less than chlorine dioxide diluted with 90%or more chlorine gas, or diluted with 90% or more of a mixture ofchlorine, carbon dioxide and water vapor.

By another broad aspect of this invention, there is provided, in aprocess for converting an aqueous solution of chlorate into chlorinedioxide by reaction thereof with hydrogen chloride gas produced in situby combustion of hydrogen gas with chlorine gas at a temperature inexcess of 600 C., the improvement which comprises cooling theso-produced hydrogen chloride gas to a temperature of 150 C. or less bydiluting said hydrogen chloride gas with suicient chlorine gas or gasmixture from chlorine dioxide generator, thereby to provide a finalgaseous reaction product comprising less than 10% chlorine dioxide 4diluted with or more chlorine gas, or diluted with 90% or more of amixture of chlorine, carbon dioxide and water vapor.

By yet another broad aspect of the present invention, a continuousprocess for the production of chlorine dioxide is provided, whichcomprises: (a) effecting electrolysis of an aqueous solution of a metalchloride whereby to form (i) an aqueous solution of a metal chlorate and(ii) gaseous hydrogen; (b) reacting gaseous hydrogen with gaseouschlorine whereby to form (iii) gaseous hydrogen chloride, (c) reactingthe aqueous solution of metal chlorate (a)(i) with gaseous hydrogenchloride from step (b)(iii) whereby to form (iv) an aqueous solution ofmetal chloride, (v) an aqueous solution of chloric acid, (d) reactingthe aqueous solution of chloric acid (c)(v) with the gaseous hydrogenchloride from step (b) (iii) whereby to form (vi) chlorine dioxide,which is recovered (vii) water and (viii) gaseous chlorine, and (e)mixing a preselected amount of gaseous chlorine with the gaseoushydrogen chloride reactant for step (d) whereby to provide a nal productfrom step (d) consisting of up to 10% chlorine dioxide and 90% or morechlorine or a mixture of chlorine, carbon dioxide and water vapor.

By yet another aspect of this invention, a closed cycle chlorine dioxidegeneration system is provided, comprising (a) an electrolytic apparatusfor the generation of an aqueous solution of chlorate (b) a generatorfor generating chlorine dioxide from said chlorate solution and hydrogenchloride (c) means connecting the liquor outlet of said electrolyticapparatus (a) with the liquor inlet of said generator (b), (d) apparatusfor the conversion of cell gases for apparatus (a) to hydrogen chloride,(e) means Connecting the gas outlet of apparatus (a) to the gas inlet ofapparatus (d), (f) means connecting the gas outlet of apparatus (d) withthe gas inlet of generator (b), (g) means for separating chlorinedioxide from gaseous chlorine, (h) means connecting the gaseous outletof generator (b) with the inlet of separator (g), (i) means connectingthe gas outlet of separator (g) to apparatus (d), and (j) meansconnecting the gas outlet of separator (g) to the gas inlet of generator(b).

The present invention therefore provides a process for the manufactureof chlorine dioxide which is based on following main reactions:(Mizrnetal ion) (l) ZMCl-i- 6H2O-il2 FARADAYS-e 2MC1O3 -i- GHZ Theaction of hydrogen chloride on the metal chlorate solution will producechlorine dioxide and chlorine according to reactions (3) and (4). Bycontrolling the acidity and by utilizing an excess of the metalchlorate, reaction (3) and (4) can be caused to yield ClO2:Cl2 in ratio2:1. Chlorine is consumed in reaction (2) for production of hydrogenchloride, thus, the system will yield chlorine dioxide only, free ofchlorine to the extent of the efficiency of chlorine dioxide gasseparator. Since the system is closed, after the initial charge ofmetallic chloride, no addition of salt is required. Furthermore, theelectrolysis in reaction (l) will produce 3 times the required amount ofhydrogen. The process of the present invention is thus based on a systemwhich requires water, chlorine and electric current for the productionof chlorine dioxide.

The present invention is basically a closed system with essentially nomajor effluent liquor other than the output of chlorine dioxidesolution. Therefore, losses of reagents are minimized and themanufacturing cost of chlorine dioxide will be determined by cost ofpower and chlorine. Thus, the system in the present invention isself-regenerating in regard to chemicals except for chlorine and water.At 100% yield, one mole of chlorine is required for production of twomoles of chlorine dioxide. The only other raw material is electricalenergy, the two moles of water per mole C102 being insignicant.

Another important aspect of the present invention is that the processyields chlorine dioxide in a safe and efficient manner. Any hydrogen gastending to remain in the chlorine dioxide generator reacts in situ withthe excess chlorine gas diluent in the chlorine dioxide generator toprovide gaseous hydrogen chloride. This gas is one of the reactants toproduce chlorine dioxide.

In addition, the chlorine dioxide gas produced in the chlorine dioxidegenerator is diluted, preferably to less than This dilution is advisablein order to minimize the risk of explosion of chlorine dioxide gas.

Another aspect of the present invention is that the process isself-contained in regard to heat insofar as it generates an excess ofheat for the process in the combustion chamber:

The temperature in the C102 generator is controlled by the heat contentof gases from the combustion chamber.

The reactant gases fed to such combustion chamber will also containexcess chlorine. Consequently the eflluent gas from the combustionchamber will not contain any excess hydrogen. This tends to make theoperating very safe and easy.

In addition, the system of this aspect of the present invention has theadvantage that the gas ow through the combustion chamber will bedetermined by the production requirements of hydrogen chloride. Thus,the temperature in the combustion chamber could be maintained high. Thetemperature in the combustion chamber will be in excess of 600 C.,usually of the order of 1000 C. This high temperature of combustion inthe combustion chamber tends to insure substantially completeutilization of the oxygen in the cell gases without employingcatalyzers. It also permits a high production capacity for any onecombustion chamber. The hot gases, namely hydrogen chloride and watervapour, leaving the combustion chamber are cooled down by the chlorinegas which is, as hereinbefore stated, recirculated for the purpose ofdiluting the product chlorine dioxide gas. Thus, the total heat value isrecovered, since there is no heat transfer. Simultaneously, the gastemperature is lowered to about 150 C. or less by such utilization ofthe cool chlorine recirculation stream as the direct contact coolingmedium by the gas recirculation to facilitate utilizing conventionaltypes of gas compressors, piping and makes the eiect of gas entrance tothe generator less violent. If it is desired to lower the temperaturefurther than achieved by the optimum recirculation rate of chlorine (andcarbon dioxide) then a heat exchanger or suitable cooling coils could beemployed in the combustion chamber or external thereto as may be thecase. The cooling may be done using the chlorate feed solution from thechlorate process or from other media.

Operating at high temperatures in the chlorine dioxide generator resultsin a substantial water evaporation and heat loss. Thus, the process ofthis aspect of the present invention, by reducing the temperature of theinput gaseous reactants, tends to minimize the water evaporation andheat losses. If the heat evolved from above combustion reaction and fromgas compressor or blower is insuicient to maintain a desirabletemperature in generator, part of the excess hydrogen can be combustedwith air in the combustion chamber according to following combustionreaction:

Another aspect of the present invention is that the process favorsoperating the electrolytic cells at a low pH. Thus, current eiciencylosses by the decomposition of hypochlorite will be minimized and thechemical attack on graphite electrodes will be less severe. The.

chlorine losses to cell gases may be higher but the chlorine issubsequently combusted with hydrogen to form hydrogen chloride and thusbenefits the production of chlorine dioxide.

Another aspect of the present invention is that the process minimizesthe dangers of explosion from chlorine dioxide by recirculating excesschlorine in order to dilute the generated gases. Designing the systemfor short gas retention time and a large surface contact area,controlling temperature of Igas mixture after generator and avoidingultraviolet light will eliminate hazards of explosions.

If, as in one aspect of the present invention, the hydro gen for theprocess is derived from the off-gases of the electrolytic chloride cell,then provision must be made to control the carbon dioxide in therecirculating stream. Carbon dioxide will otherwise accumulate,requiring increased tlow rate through the combustion chamber thusresulting in a lower combustion temperature and in incompletecombustion.

The recirculation may be utilized to control the temperature in thecombustion chamber. However, eventually some of the gas must be releasedfrom the system. Alternatively a conventional type of carbon dioxidescrubbing system is employed.

In another embodiment a second combustion chamber could be employed forrecovering the chlorine from the bleed gas and simultaneously alsoventing the excess carbon dioxide by burning the chlorine gas withexcess hydrogen from the electrolyte chlorate cell. T'he soformedhydrogen chloride may be recovered per se, may be dissolved in water toform hydrochloric acid or may be recycled to the chlorine dioxidegenerator. Alternatively, hydrogen from other sources, e.g.,chlorinealkali plants, which would be almost pure could -be employed andthus any means, such as the second combustion chamber or the CO2scrubber, would not be required. A stripper may also be provided in thesystem. This is to remove entrained chlorine and chlorine dioxide gasfrom the chlorine dioxide generator eluent liquor. Alternatively, thestripper could be employed at the generator before cooling the liquor.

In the accompanying drawings:

FIG. l is an idealized, schematic, diagrammatic representation of aprocess, including the chemical equations, of a first aspect of thepresent invention;

FIG. 2 is an idealized, schematic, diagrammatic representation of aprocess, including the chemical equations, of a second aspect of thepresent invention;

FIG. 3 is an idealized, schematic, diagrammatic representation of aprocess, includin-g the chemical equations, of a third aspect of thepresent invention;

FIG. 4 is an idealized, schematic, diagrammatic representation of aprocess, including the chemical equations, of a fourth aspect of thepresent invention;

FIG. 5 is an idealized, schematic, diagrammatic representation of aprocess, including the chemical equations, of a lifth aspect of thepresent invention;

FIG. 6 is an idealized, schematic representation of a process, includingthe chemical equations, of a sixth aspect of the present invention;

FIG. 7 is an idealized, schematic, diagrammatic representation of aprocess, including the chemical equations, of a seventh aspect of thepresent invention; and

FIG. 8 is an idealized, schematic, diagrammatic representation of aprocess, including the chemical equations, of an eighth aspect of thepresent invention.

Turning first to FIG. 1, an electrolytic cell 10, which '7 may be thatdisclosed and claimed in pending Canadian application Ser. No. 901,153tiled Apr. 24, 1964, operates to electrolize an aqueous solution of ametal chloride, for example sodium chloride. The liquid productsproceeds via line 11 to chlorine dioxide generator 12 and 13, arrangedin series to one another or combined as one unit and the gaseousproducts proceed via line 14 to a combustion chamber 15, whose purposeand function will be described hereinafter. The liquor is induced toreact in manner to be described in detail hereinafter in the chlorategenerators 12 and 13 and is recycled, after being cooled in cooler 16and line 17 to electrolytic cell 10. The effluent liquor from chlorinedioxide generator 12 and 13, in line 17, is normally high in chlorateconcentration as well as chloride.

The off-gases consisting mainly of hydrogen but including smalleramounts of other gases such as water vapour, oxygen, carbon dioxide andchlorine are burned in combustion chamber to provide hydrogen chloridegas and additional water vapour. The hot gases, usually at a temperaturein excess of 900 C. emerge from combustion chamber 15 via line 18 andare mixed and diluted with a cold, gaseous mixture of chlorine andcarbon dioxide from branch line 19 or in the combustion chamber 15 toprovide, in line 20, a wanm gas at Ia temperature of approximately 100C. consisting of hydrogen chloride, chlorine, water vapour and carbondioxide. The warm gas in line 20 is admitted to a gas compressor orblower 21 by means of which it is fed via line 22 to chlorine dioxidegenerator 12. The gaseous eluent, namely chlorine dioxide, water vapour,carbon dioxide and chlorine diluent gas, is fed via line 23 to anabsorbing tower 26. The gas leaving generator 12 in line 23 iSCl2(s)+ClO2(g)+H2O(g)+CO2*(g) (assuming the electrolytic cell employscarbon electrodes and the cell off-gas is being used forHCl-combination). The liquid elluent from chlorate generator 12 is fedvia line 24 to a second chlorine dioxide generator 13 or a chamberwithin the same generator, where additional chlorine dioxide gas isformed. The gaseous effluent consisting of chlorine dioxide gas,chlorine gas diluent and water vapour is fed via line 25 to absorbingtower 26. The liquid euent consisting of chloride liquor together withunreacted chlorate is first cooled by cooler 16 and is fed, aspreviously indicated, via line 17 back to electrolytic chlorate cell 10.

In absorbing tower 16 the gaseous eiuent from chlorine dioxidegenerators 12 and 13 is contacted with cold water entering absorbingtower via line 27. Chlorine dioxide solution is Withdrawn via line 28,and the less watersoluble gases are vented from absorbing tower 26through line 29. The chlorine dioxide solution in line 28 is alsosaturated with gaseous chlorine.

Line 29 branches into line 30 which feeds a portion of the non-absorbedgases, consisting mainly of chlorine, carbon dioxide, water vapour andcarbon dioxide to the combustion chamber 15, the amount of such gasesbeing controlled by valve 31. Line 29 also branches to line 19 where, aspreviously indicated, a cold, gaseous mixture of chlorine, carbondioxide, water vapour and carbon dioxide is added, as a diluent gas, tothe hot combustion products of combustion chamber 15. The amount of suchcold gas used as a diluent is controlled by valve 32.

Additional chlorine gas to react with the excess hydrogen in combustionchamber 15 is fed to combustion chamber 15 via line 33 controlled byvalve 34, and/or is added to the generator 12 as diluant gas. Chlorinegas admitted via line 33, may also be used to dilute gas from generator13. The gas leaving generator 13 in line 25 is otherwise high in ClO2(g)concentration.

While the process previously described is useful and eicient, carbondioxide, which is present in small amounts in the off-gases fromelectrolytic chlorate cell 10, tends to accumulate, requiring increasedow rates through the combustion chamber which also results in a lowerdegree of combustion. Accordingly, it is preferred that excess carbondioxide be removed from the closed cycle system. The embodiments shownin FIGS. 2 and 3 are two different alternative procedures for theremoval of such excess carbon dioxide.

In the description of FIGS. 2 8, which follows, only the parts of thedrawings which are different from the part previously described in FIG.l will be explicitly described, in the interest of conciseness and inorder to avoid redundancy.

In FIG. 2, the excess carbon dioxide is removed by means of a secondcombustion chamber 215. A predetermined, controlled amount of off-gasesfrom electrolytic chlorate cell 10 is fed via branch line 214 controlledby valve 217, to the second combustion chamber 215. In addition andperiodically, a bleed of the non-absorbed gases from absorbing tower 26is conducted via line 210, controlled by valve 211, to the secondcombustion chamber 215. Carbon dioxide is vented to atmosphere via stack219 and the hydrogen chloride combustion product is removed via outletline 218. The hydrogen chloride may either be recycled to outlet 18 ofcombustion chamber 15 to be used in the cyclic chlorine dioxidegeneration system or, it may be dissolved in water to form commerciallyuseful hydrochloric acid.

In the embodiment shown in FIG. 3 on the other hand, the carbon dioxideis removed by means of a conventional carbon dioxide scrubber. Thus,line 31 has disposed therein in series with its line of ow, aconventional scrubber 310 which removes the carbon dioxide from line 330which is used to feed chlorine gas diluent to the combustion chamber 15.

It is also advisable to remove entrained chlorine gas, carbon dioxidegas and chlorine dioxide gas from the chloride and chlorate liquor feedwhich is recycled from the chlorine dioxide generator 13 to theelectrolytic chlorate cell 10. This is shown in FIG. 4 as the stripper410. This stripper is a conventional one to separate the gaseousproducts which are entrained or entrapped in a liquid from the liquid.The chloride and chlorate liquor effluent is fed from stripper 410 toelectrolytic chlorate cell 10 by means of line 417. The gaseous effluentfrom the stripper 410 is fed via line 411 to off-gas line 14. A portionof the gas is then conveyed through combustion chamber 15 and theremaining portion is recycled via line 412 to stripper 410.

The embodiments shown in FIG. 5 incorporate the second combustionchamber 215 to remove excess carbon dioxide from the system (as fullydescribed hereinbefore with reference to FIG. 2), as well as thestripper to remove entrained and occluded gaseous chlorine, chlorinedioxide, and carbon dioxide (as described in greater detail hereinbeforewith reference to FIG. 4).

The embodiment in FIG. 6 shows the removal of the excess carbon dioxideby means of the scrubber 310 (previously described in greater detailwith reference to FIG. 3) and the stripper 410 to remove the entrainedand occluded gaseous chlorine, chlorine dioxide and carbon dioxide(previously described in greater detail with reference to FIG. 4).

It is also possible to avoid the build-up of carbon dioxide in thesyste-m by using pure hydrogen from other sources, such as that derivedfrom chlorine alkali plants. In such instance, it would not be necesaryto provide any means for the removal of carbon dioxide. However, it isadvisable to provide for the burning olf of the olfgases produced in theelectrolytic chlorate cell. Two such embodiments are shown in FIGS. 7and 8. The embodiment in FIG. 8 differs from the embodiment in FIG. 7only in the provision of the stripper 410 to remove entrained andoccluded gaseous chlorine and chlorine dioxide from the chloride liquorin line 17 (as previously fully described with reference to FIG. 4).

In each of FIGS. 7 and 8 the second combustion chan1- ber 215(previously fully described with reference to FIG. 2) is provided inorder to combust th olf-'gases from the electrolytic chlorate cell 10.As described with reference to FIG. 2 the carbon dioxide is Vented viastack 219 and the hydrogen chloride gas 218 may be recycled to thesystem via line 18 or may be dissolved in water to provide commerciallyuseful hydrochloric acid.

In each of FIGS. 7 and 8 the combustion chamber 15 is fed with purehydrogen through line 714.

It is noted, therefore, that the present invention provides, in all ofits embodiments a safe, easily operated, process for the continuouspreparation of chlorine dioxide in a most efficient manner.

Still another possibility is employing non-carbonaceous electrodes inthe electrolytic cell and thus eliminate carbon dioxide formation.

I claim:

1. A closed cycle system for the generation of chlorine dioxide, saidsystems comprising:

(a) an electrolytic apparatus for the generation of an aqueous solutionof chlorate, said apparatus including a liquor inlet, a liquor outletand a gas outlet;

(b) apparatus for generating chlorine dioxide from said chloratesolution and hydrogen chloride, said apparatus including a liquor inlet,a liquor outlet, a gas inlet and a gas outlet;

(c) means connecting the liquor outlet of said electrolytic apparatus(a) with the liquor inlet of said generator (b);

(d) apparatus for the conversion of cell gases for apparatus (a) tohydrogen chloride, said apparatus including gas inlet means and a \gasoutlet;

(e) means connecting the gas outlet of apparatus (a) to a gas inletmeans of apparatus (d);

(f) means connecting the gas outlet of apparatus (d) with the gas inletof apparatus (b);

(g) separator means. for separating chlorine dioxide from gaseouschlorine, said means including gas inlet means, gas outlet means, liquidinlet means and product outlet means;

(h) means connecting the gas outlet of generator (b) with a gas inletmeans of separator (g);

(i) means connecting the gas outlet means of separator (g) to a gasinlet means of apparatus (d); and

(j) means connecting a gas outlet means of separator (g) to a gas inletmeans of generator (b).

2. The system of claim 1 including a pair of said ap paratus (b) forgenerating chlorine dioxide connected together in series.

3. The system of claim 1 including (k) gas compressor or blower meansfor feeding said chlorine-diluted hydrogen chloride gas from said gasoutlet of said apparatus (d) to said gas inlet of said apparatus (b) forgenerating chlorine dioxide.

4. The system of claim 1 wherein said separator means (g) comprises anabsorbing tower for separating chlorine dioxide from the other gaseouseluent products from said apparatus (b) for generating chlorine dioxide.

5. The system of claim 4 including (l) a scrubber including gas inletmeans connected to gas outlet means from absorbing tower (g) forremoving carbon dioxide from the gaseous eluent from said absorbingtower.

6. The system of claim 4 wherein said apparatus (d) comprises a rstcombustion chamber and includes (m) a second combustion chamberincluding gas inlet means connected to gas outlet from said apparatus(a) and in parallel to said first combustion chamber for 'burning aportion of gaseous eiuent from said apparatus (a), whereby to removecarbon dioxide from the system.

7. The system of claim 6 including a line connected between gas outletmeans of absorbing tower (g) and gas inlet means of said secondcombustion chamber (In) for conducting'a portion of the gases from saidabsorbing tower (g) to said second combustion chamber (m).

8. The system of claim 1 including (n) means connecting the liquidoutlet of apparatus (b) with the liquor inlet of apparatus (a).

9. The system of claim 8 including (o) cooling means operativelyassociated with said connecting means (n).

10. The system of claim 8 including (p) means including yliquor inletmeans, liquor outlet means, gas inlet means and gas outlet meansoperatively associated with said connecting means (n) and connected toliquor outlet means from apparatus ('b) for removing occluded andentrained gases from the liquor.

11. The system of claim 10 including (q) means connecting the gaseousoutlet of said means (f) to the gas connecting means (e), and (r) secondmeans connecting said gas connecting means (e) to the gas inlet means ofsaid means (p).

12. The system of claim -9 including a pair of said apparatus (b) forgenerating chlorine dioxide connected together in series.

13. The system of claim 9 including (k) gas compressor or blower meansfor feeding chlorine-diluted hydrogen chloride gas from said gas outletof said apparatus (d) to said gas inlet of said apparatus (b) forgenerating chlorine dioxide.

14. The system of claim 9 wherein said separator means (g) comprises anabsorbing tower for separating chlorine dioxide from the other gaseouseluent products from said apparatus (b) for generating chlorine dioxide.

15. The system of claim 12 including (k) gas cornpressor or blower meansfor feeding chlorine-diluted hydrogen chloride gas from said gas outletof said apparatus (d) to said gas inlet of said apparatus (b) forgenerating chlorine dioxide, and further wherein said separator means(g) comprises an absorbing tower for separating chlorine dioxide fromthe other gaseous eilluent products from said apparatus ('b) forgenerating chlon'ne dioxide.

16. The system of claim 14 including (l) a scrubber including gas inletmeans connected to gas outlet means from absorbing tower (g) forremoving carbon dioxide from the gaseous eluent from said absorbingtower.

17. The system of claim 16 wherein said apparatus (d) comprises a firstcombustion chamber and includes (m) a second combustion chamberincluding gas inlet means connected to gas outlet from said apparatus(a) and in parallel to said first combustion chamber for burning aportion of gaseous eilluent from said apparatus (a), whereby to removecarbon dioxide from the system. i

18. The systein of claim 17 including a line connected between gasoutlet means of absorbing tower (g) and gas inlet means of said secondcombustion chamber (m) for conducting a portion of the gases from saidabsorbing tower (g) to saidsecond combustion chamber (m).

References Cited UNITED STATES PATENTS 3,404,952 10/ 1968 Westerlund23-152 JOSEPH SCOVRONEK, Primary Examiner U.S. Cl. X.R.

